BACKGROUND: Radioiodine administration is the most effective and safest treatment of hyper-thyroidism in cats. It requires only a single subcutaneous injection of 131I followed by a short period of hospitalization, the length of which is determined by local or state regulations. Administration of methimazole prior to radioiodine treatment is recommended by some clinicians so that cats have relief from the hyperthyroid state for several weeks in order to evaluate renal function and make the cats more stable medically for isolation during treatment. Methimazole acts by inhibiting thyroid peroxidase, thus reducing incorporation of iodine onto thyroglobulin and formation of thyroxine and triiodothyronine. Although methimazole does not inhibit iodine uptake by the thyroid gland, it has been thought that a rebound from recent withdrawal may alter the efficacy of radioiodine treatment.

SUMMARY: Eight 1-year-old, male, cats determined to be healthy based on physical examination, complete blood counts, and serum T4 concentration were studied. Normal values for pertechnetate (99-TcO4) and 123I were obtained in all eight and in five of the eight cats, respectively, prior to methimazole treatment. Five cats were treated with methimazole at an initial oral dose of 5 mg twice per day for one week, then the dose was increased, if necessary, to 15 mg/day until the serum T4 concentration was less than 10 ng/ml at weekly measurements. Three cats acted as untreated controls. After serum T4 con-centration was less than 10 ng/ml in all treated cats, methimazole treatment was continued for an additional three days, at which time all cats were evaluated using pertechnetate uptake studies. Images were obtained after intravenous administration of 4.94 mCi 99mTcO4 while cats were under general anesthesia. After an additional 48 hours of methimazole treatment in the treatment group, all cats underwent radioiodine uptake studies with images obtained eight and 24 hours after oral administration of 334 uCi 123I. Additional studies of radioiodine uptake and measurement of serum T4 concentration were performed 1, 4, 9, 15, and 24 days after cessation of methimazole treatment. The percent dose uptake of 99mTcO4 and 123I and the thyroid: salivary ratios of 99mTcO4 were determined. Serum T4 concentration was significantly increased after two weeks of methimazole treatment, and then decreased significantly from baseline after three weeks of treatment to a mean of 6.24 ng/ml. The serum T4 concentration returned to within the normal range two days after stopping methimazole in three cats and was normal in all five treated cats four days after cessation of treatment. Percentage uptake of 99mTcO4 and thyroid: salivary ratios were significantly increased in treated cats at all measurements after administration (20 minutes and 1, 2, and 4 hours). The percentage uptake of 123I eight hours after administration was significantly increased one to nine days after stopping methimazole treatment, with a maximum effect at four days. Percentage uptake of 123124 hours after administration was increased four to nine days after stopping methimazole, with a maximum effect at nine days. The authors concluded that methimazole treatment results in a rebound effect where radioiodine uptake is increased for four to nine days and pertechnetate scans may be enhanced for two to three weeks after discontinuing methimazole treatment.

CLINICAL IMPACT: Use of methimazole prior to radioiodine treatment has proven value in performing a trial on cats known to have hyperthyroidism and suspected to have renal failure. Radioiodine treatment is not reversible, as is methimazole dosage, and effective treatment of hyperthyroidism in a cat with concurrent renal failure may result in decreased glomerular filtration and uncompensated renal failure. The value of administering methimazole longer than to assess renal function at euthyroidism is questionable. Some centers administer radioiodine in the hopes of improving the cats condition prior to radioiodine and isolation. However, methimazole effects are gone in two days after withdrawal. Those centers which use it prior to the administration of radioiodine should withdraw it two or more weeks prior radioiodine which raises the question of methimazole treatment's value in improving speed of recovery or survival rates after radioiodine. The proposed value of methimazole treatment for the purpose of improving speed of recovery or survival after radioiodine should be critically evaluated. Based on the results of this study, methimazole treatment and recent withdrawal prior to a thyroid scan will result in enhanced uptake and an appearance of hyperthyroidism which may not exist. Methimazole treatment and recent withdrawal prior to radioiodine treatment risks uptake by normal thyroid tissue and tissue destruction that would otherwise not be affected by radioiodine. A previous study of radioiodine treatment in hyperthyroid cats noted an increased incidence of post-treatment hypothyroidism in cats in which methimazole treatment had been recently discontinued. This is consistent with the results of this study in normal cats where 99mTcO4 and radioiodine uptake are temporarily elevated for at least nine days after discontinuing methimazole treatment. Since altered radioiodine uptake was not present 15 and 24 days after stopping methimazole treatment, recommendations to discontinue this antithyroid medication for to weeks before radioiodine treatment seem reasonable. However, studies on hyperthyroid cats should be undertaken to determine if methimazole has a similar effect when the pituitary-thyroid axis is abnormal.